The invention relates to a step-down converter for converting a DC input voltage into a DC output voltage. Therein, a controlled switch and an inductance are series-connected in a series arm of the step-down converter. A freewheeling diode is arranged in a shunt arm between the controlled switch and the inductance, and a smoothing capacitor is arranged at the output side of the step-down converter. The controlled switch is controlled by a control circuit as a function of the current flowing through the series arm so that the controlled switch periodically opens and closes.
Step-down converters of this type are known in the art in many different variations. They are used, for example, for charging accumulators, for supplying power to light emitting diodes, or for supplying power to primary control circuits of switched mode power supplies.
WO 99/13559 discloses a DC/DC step-down converter that has a switching regulator. The switching regulator has switching means to switch the unregulated DC input voltage; a current-sensor-amplifier to control the switching means; a hysteresis generator to control the current-sensor-amplifier; and an output circuit to generate a regulated DC signal for the output.
If one wants to use step-down converters of the aforementioned type in a conventional domestic power system with 230 volts ACxe2x80x94or approximately 325 volts after rectificationxe2x80x94without the insertion of a transformer, a problem arises that not only the controlled switch but also the associated control circuit must have the required electric strength. Since, after power-up, there is no other voltage source other than the aforementioned high input voltage, the high input voltage must be accessed. To block one of the self-blocking field effect transistors conventionally used, the gate voltage must be turned off again, so that the full input voltage is applied. Moreover, a protective circuit for the gate must be provided in order to protect against excessively high voltages. This increases the component costs.
In another embodiment, the gate voltage is not turned off, and the gate is briefly closed. In this case, there are no special voltage requirements for the switch element, but the voltage supply for the gate must always be generated from the high input voltage. For this purpose, a series resistor is used, which, in turn, results in an undesirably high power loss. Such a series resistor must be so dimensioned that it is suitable for the high input voltage and for an increased power output. As a result, for a power-optimized series resistor, the broad input voltage range of such a step-down converter, which is normally desirable, is eliminated.
It is one object of the invention to provide a step-down converter that can be realized with a high input voltage range at low cost and with minimal power loss.
According to one formulation of the invention, this and other objects are achieved by a step-down converter for converting a DC input voltage into a DC output voltage, wherein a series arm of the step-down converter includes a controlled switch that has a self-conducting switching transistor; an inductance that is series-connected to the controlled switch; a first Zener diode; and a current sensor that senses a current flowing through the series arm. The step-down converter further includes a freewheeling diode that is arranged in a shunt arm between the controlled switch and the inductance; a capacitor parallel-connected to the first Zener diode; a smoothing capacitor arranged at an output of the step-down converter; and a control circuit having a control transistor, wherein the control transistor periodically opens and closes the controlled switch as a function of the current that flows through the series arm.
Therein, the first Zener diode and the capacitor that is parallel-connected to the first Zener diode generate a blocking voltage, which is supplied to the self-conducting switching transistor when the current sensor senses at least a pre-determined maximum value of the current.
By using a self-conducting switching transistor, the control circuit or the controlled switching element can be configured with substantially lower electric strength and is therefore less expensive. Series resistors for the power supply of the control circuit are not required, so that the associated energy dissipation is eliminated.
It is advantageous to parallel-connect a capacitor to the first Zener diode. Thereby, a blocking voltage is maintained for the field effect transistor, even if there is no current flowing anymore.
It is further advantageous to configure the switching transistor as a depletion-FET and to connect gate and drain to each other by a gate-drain resistor.
A cost-effective and simple embodiment is achieved by configuring the current sensor in the series arm as a sensor-resistor. The voltage drop occurring on the sensor-resistor can be supplied to the base-emitter path of the control transistor. Therein, the collector of the control transistor is led to the gate of the switching transistor. Advantageously, the sensor-resistor is located in the series arm between the Zener diode and the inductance.
To improve the switching behavior, it is advantageous to provide a discharge transistor, whose collector-emitter path, in series with a discharge resistor, bridges the gate-source path of the switching transistor. Therein, the voltage drop on an additional sensor-resistor in the series arm can be supplied to the base-emitter path of the discharge transistor.
To prevent damage to the smoothing capacitor if the load is eliminated, the output voltage at the smoothing capacitor is limited by a voltage limiter, such as a second Zener diode.